Differential target gene activation by the Staphylococcus aureus two-component system saeRS. J Bacteriol. 2010;192:613-623. [PMID: 19933357 DOI: 10.1128/jb.01242-09]

Assessment of the biofilm formation capacities of Staphylococcus aureus strains Newman and Newman D2C in vitro and in vivo

Staphylococcus aureus is a major cause of implant-associated infections (IAIs). The ability of this Gram-positive bacterium to cause IAIs is closely related to its capacity to attach to and to form biofilms on the implant material. Biofilm formation of S. aureus on artificial surfaces is usually mimicked in the laboratory by simple microplate-based in vitro assays and often involves type culture collection preserved laboratory strains such as SA113 (ATCC 35556), Newman (NCTC 8178), and Newman D2C (NCTC 10833, ATCC 25904). The latter two strains are phylogenetically closely related and often inadvertently indicated as strain "Newman" in publications, albeit of the fact that strain Newman D2C harbors among others mutations in the global regulatory loci agr and sae, which strongly impact the phenotypic behavior of this strain. Wondering how the genetic differences between strains Newman and Newman D2C alter the biofilm formation capacities of these two strains in vitro and in vivo, we tested here the adhesion behavior and biofilm formation capacities of both strains on different kinds of artificial surfaces (tissue culture-treated bottoms of 96-well polystyrene microplates and polyurethane-based peripheral venous catheter [PVC] tubing). Additionally, we determined their ability to cause infection in a foreign body-related murine infection model. Our studies revealed that the Newman and Newman D2C derivatives kept at Saarland University, Germany, differ significantly in their abilities to attach to microplate well bottoms and PVC tubing, and to form biofilms in various static and dynamic in vitro assays. However, when the biofilm formation capacities of both strains were determined in an in vivo infection model, rather comparable bacterial loads were observed. These findings suggest that biofilm formation capacities of S. aureus strains may differ substantially in vitro and in vivo. Additionally, researchers working with strains Newman and Newman D2C should be aware that both strains differ substantially in their phenotypic behavior, and that both strains should be indicated correctly to allow for a better comparison of data obtained with these strains in different laboratories.

Antibacterial and anti-virulence activity of eco-friendly resveratrol-loaded lipid nanocapsules against methicillin-resistant staphylococcus aureus

Methicillin-resistant Staphylococcus aureus (MRSA) is challenging modern antimicrobial therapy due to its high antimicrobial resistance. Nutraceuticals have gained a lot of interest and their incorporation into nanoparticles further improves their efficacy. This study aimed to evaluate the antibacterial activity of linalool-based lipid nanocapsules loaded with resveratrol (LIN-LNC-RES) as a synergistic strategy against MRSA. LIN-LNC-RES were prepared by the phase inversion temperature method and characterized for their colloidal properties, in vitro release, and stability. The antibacterial and antibiofilm activity against S. aureus and different MRSA clinical isolates were investigated. Furthermore, scanning electron microscopy (SEM) imaging for visualization of biofilm formation and bacterial membrane integrity as well as mechanistic investigation using quantitative real-time polymerase chain reaction (qRT-PCR) analysis were performed. LIN-LNCs-RES demonstrated favorable properties with a size of 35.19 ± 0.72 nm, PDI of 0.09 ± 0.02 and a zeta potential of -2.53 ± 0.07 mV with RES 98% EE. They showed a controlled release of RES over 24 h and were stable at 4 °C for 3 months. Compared to free drug, LIN-LNC-RES showed a 4-fold decrease in MIC values and 10-fold decrease in half maximal biofilm inhibitory concentration value. Biofilm eradication assay showed superiority of LIN-LNC-RES over RES against all isolates with disrupted bacterial membranes as revealed by SEM. Mechanistically, qRT-PCR showed that LIN-LNC-RES significantly reduced RNAIII gene expression as well as the expression of SaeRS two component system, potentially affecting quorum sensing and virulence factors expression. RES-loaded LIN-based nanosystem offers a great potential for combating MRSA infections, neutralizing its virulence activity hence, overcoming antimicrobial resistance.

Baohuoside I targets SaeR as an antivirulence strategy to disrupt MRSA biofilm formation and pathogenicity

The emergence of methicillin-resistant Staphylococcus aureus (MRSA) represents a critical global health challenge, making the SaeRS two-component system (TCS), a key regulator of S. aureus virulence, an ideal target for novel therapeutic approaches. In this study, virtual screening and thermal shift assays identified Baohuoside I (BI), a flavonol glycoside, as a potent inhibitor of the SaeR response regulator. BI significantly attenuated S. aureus pathogenicity without bactericidal effects, suppressing the expression of key virulence factors, such as hemolysin A (Hla) and Panton-Valentine leukocidin (PVL), while modulating immune evasion pathways. Additionally, BI disrupted biofilm formation, promoting the development of porous, less structured biofilms. Biochemical assays, including EMSA, CETSA, fluorescence quenching, and SPR, confirmed strong binding interactions between SaeR and BI. In vivo, BI demonstrated therapeutic efficacy in Galleria mellonella and rat MRSA models. These findings establish BI as a promising lead for nonbactericidal therapies to combat MRSA infections and mitigate resistance.

Sub-inhibitory concentrations of tigecycline could attenuate the virulence of Staphylococcus aureus by inhibiting the product of α-toxin

Staphylococcus aureus (S. aureus) infection is a serious threat to global health. This study aimed to investigate the anti-virulence efficacy of tigecycline against S. aureus. We used highly virulent S. aureus strains SA75 and JP30 to evaluate the effect of tigecycline on virulence, both of them isolated from the clinic. The MIC value of tigecycline against SA75 was 0.125 µg/mL, and that against JP30 was 0.25 µg/mL. Tigecycline did not affect the growth ability of bacteria at 0.015 µg/mL. Thus, subsequent discussions will focus on the effect of antibiotics at the latter subinhibitory concentrations that did not affect growth. First, the sub-MICs of tigecycline not only enhanced the sensitivity of S. aureus to oxidants and human whole blood but also weakened the hemolytic activity and cell adhesion level of S. aureus. Second, it undermined the survival of S. aureus in RAW264.7 and attenuated the macrophage inflammatory response induced by S. aureus. On the contrary, tigecycline decreased the hemolytic activity, as well as the skin abscess formation and bacterial burden in mice. Most importantly, it significantly decreased the expression of hla, hlgB, hlgC, spa, sbi, saeR, sak, tst, and coa genes by RT-qPCR and the protein expression of α-toxin. Altogether, the sub-MICs of tigecycline might be a promising agent to attenuate the virulence of S. aureus and its host immune response by inhibiting the SaeRS two-component system and the product of α-toxin.IMPORTANCEIn this study, the sub-MICs of tigecycline decreased the resistance of S. aureus to oxidants and human whole blood. Moreover, tigecycline weakened the cell adhesion level of S. aureus and skin abscess formation in mice by reducing bacterial burden. Remarkably, tigecycline decreased the hemolytic activity and significantly downregulated the expression of various virulence genes and α-toxin. This research highlighted that the sub-MICs of tigecycline might be a promising agent to attenuate the virulence of S. aureus by inhibiting the product of α-toxin.

Tetracycline and Oxacillin Act Synergistically on Biofilms and Display Increased Efficacy In Vivo Against Staphylococcus aureus

Oxacillin (bactericidal) and tetracycline (bacteriostatic) are clinically relevant antibiotics that are routinely prescribed to treat Staphylococcus aureus infections but not conventionally used in combination. There is an urgent need for treatment regimens that can act upon biofilms during infection, associated with chronic infections on indwelling devices, as well as acute planktonic (systemic) infection. Here we show that in an in vitro model oxacillin and tetracycline act synergistically against S. aureus UAMS-1 biofilms, reducing the concentration of both antibiotics necessary to eradicate an established biofilm. Using an in vivo zebrafish larval infection model with S. aureus NewHG, they display improved bacterial clearance compared to each drug alone and can counteract a loss of host phagocytes, an important innate defence against S. aureus. In these cases, the bacteriostatic nature of tetracycline enhances rather than dampens the bactericidal action of oxacillin, although an exact mechanism remains to be elucidated. We suggest a dual therapy could be of clinical use against biofilm-forming S. aureus and has a potential use in patients with a compromised immune system.

Staphylococcus aureus SaeRS impairs macrophage immune functions through bacterial clumps formation in the early stage of infection

The Staphylococcus aureus (S. aureus) SaeRS two-component system (TCS) regulates over 20 virulence factors. While its impact on chronic infection has been thoroughly discussed, its role in the early stage of infection remains elusive. Since macrophages serve as the primary immune defenders at the onset of infection, this study investigates the influence of SaeRS on macrophage functions and elucidates the underlying mechanisms. Macrophage expression of inflammatory and chemotactic factors, phagocytosis, and bactericidal activity against S. aureus were assessed, along with the evaluation of cellular oxidative stress. SaeRS was found to impair macrophage function. Mechanistically, SaeRS inhibited NF-κB pathway activation via toll-like receptor 2 (TLR2). Its immune-modulating effect could partially be explained by the strengthened biofilm formation. More importantly, we found SaeRS compromised macrophage immune functions at early infection stages even prior to biofilm formation. These early immune evasion effects were dependent on bacterial clumping as cytokine secretion, phagocytosis, and bactericidal activity were repaired when clumping was inhibited. We speculate that the bacterial clumping-mediated antigen mask is responsible for SaeRS-mediated immune evasion at the early infection stage. In vivo, ΔsaeRS infection was cleared earlier, accompanied by early pro-inflammatory cytokines production, and increased tissue oxidative stress. Subsequently, macrophages transitioned to an anti-inflammatory state, thereby promoting tissue repair. In summary, our findings underscore the critical role of the SaeRS TCS in S. aureus pathogenicity, particularly during early infection, which is likely initiated by SaeRS-mediated bacterial clumping.

Phylogenetic analysis and virulence characteristics of methicillin-resistant Staphylococcus aureus ST764-SCCmec II: an emerging hypervirulent clone ST764-t1084 in China

Previous studies have shown that the increased prevalent ST764 clone in China, Japan, and other Asian areas. However, the knowledge of the genetic features and virulence characteristics of methicillin-resistant Staphylococcus aureus (MRSA) ST764 in China is still limited. In this study, we identified 52 ST764-SCCmec type II isolates collected from five cities in China between 2014 and 2021. Whole genome sequencing showed that the most common staphylococcal protein A (spa) types of ST764 in China were t002 (55.78%) and t1084 (40.38%). Virulence assays showed that ST764-t1084 isolates had high haemolytic activity and α-toxin levels. Of the critical regulatory factors affecting α-toxin production, only the SaeRS was highly expressed in ST764-t1084 isolates. Mouse abscess model indicated that the virulence of ST764-t1084 isolates was comparable to that of S. aureus USA300-LAC famous for its hypervirulence. Interestingly, ST764-t002 isolates exhibited stronger biofilm formation and cell adhesion capacities than ST764-t1084 isolates. This seems to explain why ST764-t002 subclone has become more prevalent in China in recent years. Phylogenetic analysis suggested that all ST764 isolates from China in Clade III were closely related to KUN1163 (an isolate from Japan). Notably, genomic analysis revealed that the 52 ST764 isolates did not carry arginine catabolic mobile element (ACME), which differed from ST764 isolates in Japan. Additionally, most ST764 isolates (69.23%) harboured an obvious deletion of approximately 5 kb in the SCCmec II cassette region compared to KUN1163. Our findings shed light on the potential global transmission and genotypic as well as phenotypic characteristics of ST764 lineage.

SaeR as a novel target for antivirulence therapy against Staphylococcus aureus

Staphylococcus aureus is a major human pathogen responsible for a wide range of clinical infections. SaeRS is one of the two-component systems in S. aureus that modulate multiple virulence factors. Although SaeR is required for S. aureus to develop an infection, inhibitors have not been reported. Using an in vivo knockdown method, we demonstrated that SaeR is targetable for the discovery of antivirulence agent. HR3744 was discovered through a high-throughput screening utilizing a GFP-Lux dual reporter system driven by saeP1 promoter. The antivirulence efficacy of HR3744 was tested using Western blot, Quantitative Polymerase Chain Reaction, leucotoxicity, and haemolysis tests. In electrophoresis mobility shift assay, HR3744 inhibited SaeR-DNA probe binding. WaterLOGSY-NMR test showed HR3744 directly interacted with SaeR's DNA-binding domain. When SaeR was deleted, HR3744 lost its antivirulence property, validating the target specificity. Virtual docking and mutagenesis were used to confirm the target's specificity. When Glu159 was changed to Asn, the bacteria developed resistance to HR3744. A structure-activity relationship study revealed that a molecule with a slight modification did not inhibit SaeR, indicating the selectivity of HR3744. Interestingly, we found that SAV13, an analogue of HR3744, was four times more potent than HR3744 and demonstrated identical antivirulence properties and target specificity. In a mouse bacteraemia model, both HR3744 and SAV13 exhibited in vivo effectiveness. Collectively, we identified the first SaeR inhibitor, which exhibited in vitro and in vivo antivirulence properties, and proved that SaeR could be a novel target for developing antivirulence drugs against S. aureus infections.

Transcription factor expression levels and environmental signals constrain transcription factor innovation

Evolutionary innovation of transcription factors frequently drives phenotypic diversification and adaptation to environmental change. Transcription factors can gain or lose connections to target genes, resulting in novel regulatory responses and phenotypes. However the frequency of functional adaptation varies between different regulators, even when they are closely related. To identify factors influencing propensity for innovation, we utilise a Pseudomonas fluorescens SBW25 strain rendered incapable of flagellar mediated motility in soft-agar plates via deletion of the flagellar master regulator (fleQ ). This bacterium can evolve to rescue flagellar motility via gene regulatory network rewiring of an alternative transcription factor to rescue activity of FleQ. Previously, we have identified two members (out of 22) of the RpoN-dependent enhancer binding protein (RpoN-EBP) family of transcription factors (NtrC and PFLU1132) that are capable of innovating in this way. These two transcription factors rescue motility repeatably and reliably in a strict hierarchy – with NtrC the only route in a ∆fleQ background, and PFLU1132 the only route in a ∆fleQ ∆ntrC background. However, why other members in the same transcription factor family have not been observed to rescue flagellar activity is unclear. Previous work shows that protein homology cannot explain this pattern within the protein family (RpoN-EBPs), and mutations in strains that rescued motility suggested high levels of transcription factor expression and activation drive innovation. We predict that mutations that increase expression of the transcription factor are vital to unlock evolutionary potential for innovation. Here, we construct titratable expression mutant lines for 11 of the RpoN-EBPs in P. fluorescens . We show that in five additional RpoN-EBPs (FleR, HbcR, GcsR, DctD, AauR and PFLU2209), high expression levels result in different mutations conferring motility rescue, suggesting alternative rewiring pathways. Our results indicate that expression levels (and not protein homology) of RpoN-EBPs are a key constraining factor in determining evolutionary potential for innovation. This suggests that transcription factors that can achieve high expression through few mutational changes, or transcription factors that are active in the selective environment, are more likely to innovate and contribute to adaptive gene regulatory network evolution.

A genetic regulatory see-saw of biofilm and virulence in MRSA pathogenesis

Staphylococcus aureus is one of the most common opportunistic human pathogens causing several infectious diseases. Ever since the emergence of the first methicillin-resistant Staphylococcus aureus (MRSA) strain decades back, the organism has been a major cause of hospital-acquired infections (HA-MRSA). The spread of this pathogen across the community led to the emergence of a more virulent subtype of the strain, i.e., Community acquired Methicillin resistant Staphylococcus aureus (CA-MRSA). Hence, WHO has declared Staphylococcus aureus as a high-priority pathogen. MRSA pathogenesis is remarkable because of the ability of this "superbug" to form robust biofilm both in vivo and in vitro by the formation of polysaccharide intercellular adhesin (PIA), extracellular DNA (eDNA), wall teichoic acids (WTAs), and capsule (CP), which are major components that impart stability to a biofilm. On the other hand, secretion of a diverse array of virulence factors such as hemolysins, leukotoxins, enterotoxins, and Protein A regulated by agr and sae two-component systems (TCS) aids in combating host immune response. The up- and downregulation of adhesion genes involved in biofilm formation and genes responsible for synthesizing virulence factors during different stages of infection act as a genetic regulatory see-saw in the pathogenesis of MRSA. This review provides insight into the evolution and pathogenesis of MRSA infections with a focus on genetic regulation of biofilm formation and virulence factors secretion.

The Small Protein ScrA Influences Staphylococcus aureus Virulence-Related Processes via the SaeRS System

Staphylococcus aureus is a Gram-positive commensal and opportunistic pathogen able to cause diseases ranging from mild skin infections to life-threatening endocarditis and toxic shock syndrome. The ability to cause such an array of diseases is due to the complex S. aureus regulatory network controlling an assortment of virulence factors, including adhesins, hemolysins, proteases, and lipases. This regulatory network is controlled by both protein and RNA elements. We previously identified a novel regulatory protein called ScrA, which, when overexpressed, leads to the increased activity and expression of the SaeRS regulon. In this study, we further explore the role of ScrA and examine the consequences to the bacterial cell of scrA gene disruption. These results demonstrate that scrA is required for several virulence-related processes, and in many cases, the phenotypes of the scrA mutant are inverse to those observed in cells overexpressing ScrA. Interestingly, while the majority of ScrA-mediated phenotypes appear to rely on the SaeRS system, our results also indicate that ScrA may also act independently of SaeRS when regulating hemolytic activity. Finally, using a murine model of infection, we demonstrate that scrA is required for virulence, potentially in an organ-specific manner. IMPORTANCE Staphylococcus aureus is the cause of several potentially life-threatening infections. An assortment of toxins and virulence factors allows such a wide range of infections. However, an assortment of toxins or virulence factors requires complex regulation to control expression under all of the different conditions encountered by the bacterium. Understanding the intricate web of regulatory systems allows the development of novel approaches to combat S. aureus infections. Here, we have shown that the small protein ScrA, which was previously identified by our laboratory, influences several virulence-related functions through the SaeRS global regulatory system. These findings add ScrA to the growing list of virulence regulators in S. aureus.

Phenotypes controlled by the Brucella abortus two component system BvrR/BvrS are differentially impacted by BvrR phosphorylation

Brucella abortus is a zoonotic pathogen whose virulence depends on its ability to survive intracellularly at the endoplasmic reticulum derived compartment. The two-component system BvrR/BvrS (BvrRS) is essential for intracellular survival due to the transcriptional control of the type IV secretion system VirB and its transcriptional regulator VjbR. It is a master regulator of several traits including membrane homeostasis by controlling gene expression of membrane components, such as Omp25. BvrR phosphorylation is related to DNA binding at target regions, thereby repressing or activating gene transcription. To understand the role of BvrR phosphorylation we generated dominant positive and negative versions of this response regulator, mimicking phosphorylated and non-phosphorylated BvrR states and, in addition to the wild-type version, these variants were introduced in a BvrR negative background. We then characterized BvrRS-controlled phenotypes and assessed the expression of proteins regulated by the system. We found two regulatory patterns exerted by BvrR. The first pattern was represented by resistance to polymyxin and expression of Omp25 (membrane conformation) which were restored to normal levels by the dominant positive and the wild-type version, but not the dominant negative BvrR. The second pattern was represented by intracellular survival and expression of VjbR and VirB (virulence) which were, again, complemented by the wild-type and the dominant positive variants of BvrR but were also significantly restored by complementation with the dominant negative BvrR. These results indicate a differential transcriptional response of the genes controlled to the phosphorylation status of BvrR and suggest that unphosphorylated BvrR binds and impacts the expression of a subset of genes. We confirmed this hypothesis by showing that the dominant negative BvrR did not interact with the omp25 promoter whereas it could interact with vjbR promoter. Furthermore, a global transcriptional analysis revealed that a subset of genes responds to the presence of the dominant negative BvrR. Thus, BvrR possesses diverse strategies to exert transcriptional control on the genes it regulates and, consequently, impacting on the phenotypes controlled by this response regulator.

Disturbing the Spatial Organization of Biofilm Communities Affects Expression of agr-Regulated Virulence Factors in Staphylococcus aureus

Staphylococcus aureus uses quorum sensing and nutrient availability to control the expression of agr-regulated virulence factors. Quorum sensing is mediated by autoinducing peptide (AIP), which at a high concentration reduces expression of surface attachment proteins (coa, fnbpA) and increases expression of exotoxins (lukS) and proteases (splA). Nutrient availability can be sensed through the saeS/saeR system. Low nutrients increase expression of saeR, which augments expression of coa and fnbpA, distinct from the activity of AIP. The formation of spatial structure, such as biofilms, can alter quorum sensing and nutrient acquisition. In natural environments, biofilms encounter forces that may alter their spatial structure. These forces may impact quorum sensing and/or nutrient acquisition and thus affect the expression of agr-regulated virulence factors. However, this has not been studied. We show that periodically disturbing biofilms composed of S. aureus using a physical force affected the expression of agr-regulated virulence factors. In nutrient-poor environments, disturbance increased the expression of coa, fnbpA, lukS, and splA. Disturbance in a nutrient-rich environment at low or high disturbance amplitudes moderately reduced expression of coa and fnbpA but increased expression of lukS and splA. Interestingly, at an intermediate amplitude, the overall expression of agr-regulated virulence factors was the lowest; expression of lukS and splA remained unchanged relative to an undisturbed biofilm, while expression of coa and fnbpA significantly decreased. We hypothesize that these changes are a result of disturbance-driven changes in access to AIP and nutrients. Our results may allow the identification of environments where virulence is enhanced, or reduced, owing to a disturbance. IMPORTANCE Bacteria, such as Staphylococcus aureus, integrate signals from the environment to regulate genes encoding virulence factors. These signals include those produced by quorum-sensing systems and nutrient availability. We show that disturbing the spatial organization of S. aureus populations can lead to changes in the expression of virulence factors, likely by altering the ways in which S. aureus detects these signals. Our work may allow us to identify environments that increase or reduce the expression of virulence factors in S. aureus.

Staphylococcus aureus binding to Seraph® 100 Microbind® Affinity Filter: Effects of surface protein expression and treatment duration

INTRODUCTION

Extracorporeal blood purification systems represent a promising alternative for treatment of blood stream infections with multiresistant bacteria.

OBJECTIVES

The aim of this study was to analyse the binding activity of S. aureus to Seraph affinity filters based on heparin coated beads and to identify effectors influencing this binding activity.

RESULTS

To test the binding activity, we used gfp-expressing S. aureus Newman strains inoculated either in 0.9% NaCl or in blood plasma and determined the number of unbound bacteria by FACS analyses after passing through Seraph affinity filters. The binding activity of S. aureus was clearly impaired in human plasma: while a percent removal of 42% was observed in 0.9% NaCl (p-value 0.0472) using Seraph mini columns, a percent removal of only 10% was achieved in human plasma (p-value 0.0934). The different composition of surface proteins in S. aureus caused by the loss of SarA, SigB, Lgt, and SaeS had no significant influence on its binding activity. In a clinically relevant approach using the Seraph® 100 Microbind® Affinity Filter and 1000 ml of human blood plasma from four different donors, the duration of treatment was shown to have a critical effect on the rate of bacterial reduction. Within the first four hours, the number of bacteria decreased continuously and the reduction in bacteria reached statistical significance after two hours of treatment (percentage reduction 64%, p-value 0.01165). The final reduction after four hours of treatment was close to 90% and is dependent on donor. The capacity of Seraph® 100 for S. aureus in human plasma was approximately 5 x 108 cells.

CONCLUSIONS

The Seraph affinity filter, based on heparin-coated beads, is a highly efficient method for reducing S. aureus in human blood plasma, with efficiency dependent on blood plasma composition and treatment duration.

PurN Is Involved in Antibiotic Tolerance and Virulence in Staphylococcus aureus

Staphylococcus aureus can cause chronic infections which are closely related to persister formation. Purine metabolism is involved in S. aureus persister formation, and purN, encoding phosphoribosylglycinamide formyltransferase, is an important gene in the purine metabolism process. In this study, we generated a ΔpurN mutant of the S. aureus Newman strain and assessed its roles in antibiotic tolerance and virulence. The ΔpurN in the late exponential phase had a significant defect in persistence to antibiotics. Complementation of the ΔpurN restored its tolerance to different antibiotics. PurN significantly affected virulence gene expression, hemolytic ability, and biofilm formation in S. aureus. Moreover, the LD₅₀ (3.28 × 10¹⁰ CFU/mL) of the ΔpurN for BALB/c mice was significantly higher than that of the parental strain (2.81 × 10⁹ CFU/mL). Transcriptome analysis revealed that 58 genes that were involved in purine metabolism, alanine, aspartate, glutamate metabolism, and 2-oxocarboxylic acid metabolism, etc., were downregulated, while 24 genes involved in ABC transporter and transferase activity were upregulated in ΔpurN vs. parental strain. Protein-protein interaction network showed that there was a close relationship between PurN and GltB, and SaeRS. The study demonstrated that PurN participates in the formation of the late exponential phase S. aureus persisters via GltB and regulates its virulence by activating the SaeRS two-component system.

Regulation of the Sae Two-Component System by Branched-Chain Fatty Acids in Staphylococcus aureus

Staphylococcus aureus is a ubiquitous Gram-positive bacterium and an opportunistic human pathogen. S. aureus pathogenesis relies on a complex network of regulatory factors that adjust gene expression. Two important factors in this network are CodY, a repressor protein responsive to nutrient availability, and the SaeRS two-component system (TCS), which responds to neutrophil-produced factors. Our previous work revealed that CodY regulates the secretion of many toxins indirectly via Sae through an unknown mechanism. We report that disruption of codY results in increased levels of phosphorylated SaeR (SaeR~P) and that codY mutant cell membranes contain a higher percentage of branched-chain fatty acids (BCFAs) than do wild-type membranes, prompting us to hypothesize that changes to membrane composition modulate the activity of the SaeS sensor kinase. Disrupting the lpdA gene encoding dihydrolipoyl dehydrogenase, which is critical for BCFA synthesis, significantly reduced the abundance of SaeR, phosphorylated SaeR, and BCFAs in the membrane, resulting in reduced toxin production and attenuated virulence. Lower SaeR levels could be explained in part by reduced stability. Sae activity in the lpdA mutant could be complemented genetically and chemically with exogenous short- or full-length BCFAs. Intriguingly, lack of lpdA also alters the activity of other TCSs, suggesting a specific BCFA requirement managing the basal activity of multiple TCSs. These results reveal a novel method of posttranscriptional virulence regulation via BCFA synthesis, potentially linking CodY activity to multiple virulence regulators in S. aureus. IMPORTANCE Two-component systems (TCSs) are an essential way that bacteria sense and respond to their environment. These systems are usually composed of a membrane-bound histidine kinase that phosphorylates a cytoplasmic response regulator. Because most of the histidine kinases are embedded in the membrane, lipids can allosterically regulate the activity of these sensors. In this study, we reveal that branched-chain fatty acids (BCFAs) are required for the activation of multiple TCSs in Staphylococcus aureus. Using both genetic and biochemical data, we show that the activity of the virulence activator SaeS and the phosphorylation of its response regulator SaeR are reduced in a branched-chain keto-acid dehydrogenase complex mutant and that defects in BCFA synthesis have far-reaching consequences for exotoxin secretion and virulence. Finally, we show that mutation of the global nutritional regulator CodY alters BCFA content in the membrane, revealing a potential mechanism of posttranscriptional regulation of the Sae system by CodY.

Glucose Mediates Niche-Specific Repression of Staphylococcus aureus Toxic Shock Syndrome Toxin-1 through the Activity of CcpA in the Vaginal Environment

Staphylococcus aureus chronically colonizes up to 30% of the human population on the skin or mucous membranes, including the nasal tract or vaginal canal. While colonization is often benign, this bacterium also has the capability to cause serious infections. Menstrual toxic shock syndrome (mTSS) is a serious toxinosis associated with improper use of tampons, which can induce an environment that is favorable to the production of the superantigen known as toxic shock syndrome toxin-1 (TSST-1). To better understand environmental signaling that influences TSST-1 production, we analyzed expression in the prototype mTSS strain S. aureus MN8. Using transcriptional and protein-based analysis in two niche-related media, we observed that TSST-1 expression was significantly higher in synthetic nasal medium (SNM) than in vaginally defined medium (VDM). One major divergence in medium composition was high glucose concentration in VDM. The glucose-dependent virulence regulator gene ccpA was deleted in MN8, and, compared with wild-type MN8, we observed increased TSST-1 expression in the ΔccpA mutant when grown in VDM, suggesting that TSST-1 is repressed by catabolite control protein A (CcpA) in the vaginal environment. We were able to relieve CcpA-mediated repression by modifying the glucose level in vaginal conditions, confirming that changes in nutritional conditions contribute to the overexpression of TSST-1 that can lead to mTSS. We also compared CcpA-mediated repression to other key regulators of tst, finding that CcpA regulation is dominant compared to other characterized regulatory mechanisms. This study underlines the importance of environmental signaling for S. aureus pathogenesis in the context of mTSS. IMPORTANCE Menstrual toxic shock syndrome (mTSS) is caused by strains of Staphylococcus aureus that overproduce a toxin known as toxic shock syndrome toxin-1 (TSST-1). This work studied how glucose levels in a model vaginal environment could influence the amount of TSST-1 that is produced by S. aureus. We found that high levels of glucose repress TSST-1 production, and this is done by a regulatory protein called catabolite control protein A (CcpA). The research also demonstrated that, compared with other regulatory proteins, the CcpA regulator appears to be the most important for maintaining low levels of TSST-1 in the vaginal environment, and this information helps to understand how changes in the vaginal environmental can lead to mTSS.

Small-molecule compound SYG-180-2-2 attenuates Staphylococcus aureus virulence by inhibiting hemolysin and staphyloxanthin production

Multi-drug resistant Staphylococcus aureus infection is still a serious threat to global health. Therefore, there is an urgent need to develop new antibacterial agents based on virulence factor therapy to overcome drug resistance. Previously, we synthesized SYG-180-2-2 (C21H16N2OSe), an effective small molecule compound against biofilm. The aim of this study was to investigate the anti-virulence efficacy of SYG-180-2-2 against Staphylococcus aureus. MIC results demonstrated no apparent antibacterial activity of the SYG-180-2-2. The growth curve assay showed that SYG-180-2-2 had nonlethal effect on S. aureus. Besides, SYG-180-2-2 strongly inhibited the hemolytic activity and staphyloxanthin synthesis in S. aureus. Inhibition of staphyloxanthin by SYG-180-2-2 enhanced the sensitivity of S. aureus to oxidants and human whole blood. In addition, SYG-180-2-2 significantly decreased the expression of saeR-mediated hemolytic gene hlb and staphyloxanthin-related crtM, crtN and sigB genes by quantitative polymerase chain reaction (qPCR). Meanwhile, the expression of oxidative stress-related genes sodA, sodM and katA also decreased. Galleria Mellonella assay revealed that SYG-180-2-2 was not toxic to larvae. Further, the larvae infection model showed that the virulence of bacteria was significantly reduced after 4 μg/mL SYG-180-2-2 treatment. SYG-180-2-2 also reduced skin abscess formation in mice by reducing bacterial burden and subcutaneous inflammation. In conclusion, SYG-180-2-2 might be a promising agent to attenuate the virulence of S. aureus by targeting genes associated with hemolytic activity and staphyloxanthin synthesis.

Air pollution induces Staphylococcus aureus USA300 respiratory tract colonization mediated by specific bacterial genetic responses involving the global virulence gene regulators Agr and Sae

Exposure to particulate matter (PM), a major component of air pollution, is associated with exacerbation of chronic respiratory disease, and infectious diseases such as community-acquired pneumonia. Although PM can cause adverse health effects through direct damage to host cells, our previous study showed that PM can also impact bacterial behaviour by promoting in vivo colonization. In this study we describe the genetic mechanisms involved in the bacterial response to exposure to black carbon (BC), a constituent of PM found in most sources of air pollution. We show that Staphylococcus aureus strain USA300 LAC grown in BC prior to inoculation showed increased murine respiratory tract colonization and pulmonary invasion in vivo, as well as adhesion and invasion of human epithelial cells in vitro. Global transcriptional analysis showed that BC has a widespread effect on S. aureus transcriptional responses, altering the regulation of the major virulence gene regulators Sae and Agr and causing increased expression of genes encoding toxins, proteases and immune evasion factors. Together these data describe a previously unrecognized causative mechanism of air pollution-associated infection, in that exposure to BC can increase bacterial colonization and virulence factor expression by acting directly on the bacterium rather than via the host.

Brominated Carbazole with Antibiotic Adjuvant Activity Displays Pleiotropic Effects in MRSA's Transcriptome

Methicillin-resistant Staphylococcus aureus (MRSA) is a major threat to human health, as the US mortality rate outweighs those from HIV, tuberculosis, and viral hepatitis combined. In the wake of the COVID-19 pandemic, antibiotic-resistant bacterial infections acquired during hospital stays have increased. Antibiotic adjuvants are a key strategy to combat these bacteria. We have evaluated several small molecule antibiotic adjuvants that have strong potentiation with β-lactam antibiotics and are likely inhibiting a master regulatory kinase, Stk1. Here, we investigated how the lead adjuvant (compound 8) exerts its effects in a more comprehensive manner. We hypothesized that the expression levels of key resistance genes would decrease once cotreated with oxacillin and the adjuvant. Furthermore, bioinformatic analyses would reveal biochemical pathways enriched in differentially expressed genes. RNA-seq analysis showed 176 and 233 genes significantly up- and downregulated, respectively, in response to cotreatment. Gene ontology categories and biochemical pathways that were significantly enriched with downregulated genes involved carbohydrate utilization, such as the citrate cycle and the phosphotransferase system. One of the most populated pathways was S. aureus infection. Results from an interaction network constructed with affected gene products supported the hypothesis that Stk1 is a target of compound 8. This study revealed a dramatic impact of our lead adjuvant on the transcriptome that is consistent with a pleiotropic effect due to Stk1 inhibition. These results point to this antibiotic adjuvant having potential broad therapeutic use in combatting MRSA.

The anti-infective activity of Salvia miltiorrhiza against Staphylococcus aureus by attenuating accessory gene regulator system-mediated virulence

Due to the rapid evolution of antibiotic resistance in Staphylococcus aureus, antivirulence therapy may be a promising alternative for the effective control of the spread of resistant pathogens. The Chinese Materia Medica has been widely used for the treatment of diseases and production of health foods, and it remains a valuable resource for the discovery of compounds possessing antivirulence activity. Through a Caenorhabditis elegans infection model, an EtOAc-soluble fraction of 80% EtOH extract of Salvia miltiorrhiza Bunge (SMEA) was found to possess potential anti-infective activity against S. aureus. Then, several in vitro assays indicated that SMEA had robust antivirulence activity at the dose of 400 μg mL^-1, reducing hemolytic activity and α-hemolysin expression in S. aureus. Furthermore, at 100 mg kg^-1, SMEA reduced abscess formation in the main organs of mice challenged with S. aureus. In order to identify the bioactive components of SMEA and investigate the mechanisms underlying the antivirulence activity, SMEA was separated using bioassay-guided fractionation. As a result, eight compounds were identified in SMEA. Among them, tanshinone IIB (TNB) showed strong antivirulence activity both in vitro and in vivo. Furthermore, at 24 μg mL^-1, TNB significantly reduced the expression of RNAIII and psmα, indicating that the mechanism underlying TNB activity was related to the accessory gene regulator quorum sensing system. In conclusion, TNB's antivirulence properties make it a promising candidate for drug development against S. aureus infections.

VraSR Regulatory System Contributes to the Virulence of Community-Associated Methicillin-Resistant Staphylococcus aureus (CA-MRSA) in a 3D-Skin Model and Skin Infection of Humanized Mouse Model

The vancomycin-resistance associated sensor/regulator, VraSR two-component regulatory-system (VraSR), regulates virulence and the response of Staphylococcus aureus (SA) to environmental stress. To investigate the role of VraSR in SA skin and soft tissue infections (SSTI), we inactivated the VraSR of a clinical CA-MRSA ST30 strain by insertional mutation in vraR gene using the TargeTron-Gene Knockout System. We constructed an organotypic keratinocyte fibroblast co-culture (3D-skin model) and a humanized mouse as SSTI infection models. In the 3D-skin model, inactivation of VraSR in the strains ST30 and USA300 showed 1-log reduction in adhesion and internalization (p < 0.001) compared to the respective wildtype. The mutant strains of ST30 (p < 0.05) and USA300-LAC (p < 0.001) also exhibited reduced apoptosis. The wildtype ST30 infection in the humanized mouse model demonstrated increased skin lesion size and bacterial burden compared to BALB/c mice (p < 0.01). The response of the humanized mouse towards the MRSA infection exhibited human similarity indicating that the humanized mouse SSTI model is more suitable for evaluating the role of virulence determinants. Inactivation of VraSR in ST30 strain resulted in decreased skin lesion size in the humanized mouse SSTI model (p < 0.05) and reduction in apoptotic index (p < 0.01) when compared with the wildtype. Our results reveal that inactivating the VraSR system may be a potent anti-virulence approach to control MRSA infection.

Two-Component Systems of S. aureus: Signaling and Sensing Mechanisms

Staphylococcus aureus encodes 16 two-component systems (TCSs) that enable the bacteria to sense and respond to changing environmental conditions. Considering the function of these TCSs in bacterial survival and their potential role as drug targets, it is important to understand the exact mechanisms underlying signal perception. The differences between the sensing of appropriate signals and the transcriptional activation of the TCS system are often not well described, and the signaling mechanisms are only partially understood. Here, we review present insights into which signals are sensed by histidine kinases in S. aureus to promote appropriate gene expression in response to diverse environmental challenges.

The Sensor Histidine Kinase ArlS Is Necessary for Staphylococcus aureus To Activate ArlR in Response to Nutrient Availability

Staphylococcus aureus is a versatile opportunistic pathogen whose success is driven by its ability to adapt to diverse environments and host-imposed stresses. Two-component signal transduction systems, such as ArlRS, often mediate these adaptations. Loss of ArlRS or the response regulator ArlR alone impairs the ability of S. aureus to respond to host-imposed manganese starvation and glucose limitation. As sensor histidine kinases and response regulators frequently work as pairs, it has been assumed that ArlS senses and activates ArlR in response to these stimuli. However, recent work suggests that the sensor histidine kinase GraS can also activate ArlR, calling the contribution of ArlS in responding to manganese and glucose availability into question. The results of current studies reveal that ArlS is necessary to activate ArlR in response to manganese sequestration by the host immune effector calprotectin and glucose limitation. Although the loss of ArlS does not completely eliminate ArlR activity, this response regulator is no longer responsive to manganese or glucose availability in the absence of its cognate histidine kinase. Despite the residual activity of ArlR in the absence of ArlS, ArlR phosphorylation by ArlS is required for S. aureus to resist calprotectin-imposed metal starvation. Cumulatively, these findings contribute to the understanding of S. aureus signal transduction in response to nutritional immunity and support the previous observation indicating that ArlRS is activated by a common signal derived from host-imposed manganese and glucose limitation. IMPORTANCE The ability of pathogens, including Staphylococcus aureus, to sense and adapt to diverse environments partially relies on two-component systems, such as ArlRS. Recent work revealed that the response regulator ArlR can be cross-activated by the sensor histidine kinase GraS, rendering the role of its cognate partner, ArlS, in response to manganese and glucose limitation uncertain. The results of this study reveal that ArlS is necessary for the activation of ArlR in response to calprotectin and glucose limitation. Although a low level of ArlR activity remains in the absence of ArlS, ArlS phosphotransfer to ArlR is required for S. aureus to overcome calprotectin-induced nutritional stress. Collectively, this study provides fundamental information to understand how ArlRS mediates staphylococcal adaptation during infection.

Staphylococcusaureus Transcriptome Data and Metabolic Modelling Investigate the Interplay of Ser/Thr Kinase PknB, Its Phosphatase Stp, the glmR/yvcK Regulon and the cdaA Operon for Metabolic Adaptation

Serine/threonine kinase PknB and its corresponding phosphatase Stp are important regulators of many cell functions in the pathogen S. aureus. Genome-scale gene expression data of S. aureus strain NewHG (sigB⁺) elucidated their effect on physiological functions. Moreover, metabolic modelling from these data inferred metabolic adaptations. We compared wild-type to deletion strains lacking pknB, stp or both. Ser/Thr phosphorylation of target proteins by PknB switched amino acid catabolism off and gluconeogenesis on to provide the cell with sufficient components. We revealed a significant impact of PknB and Stp on peptidoglycan, nucleotide and aromatic amino acid synthesis, as well as catabolism involving aspartate transaminase. Moreover, pyrimidine synthesis was dramatically impaired by stp deletion but only slightly by functional loss of PknB. In double knockouts, higher activity concerned genes involved in peptidoglycan, purine and aromatic amino acid synthesis from glucose but lower activity of pyrimidine synthesis from glucose compared to the wild type. A second transcriptome dataset from S. aureus NCTC 8325 (sigB⁻) validated the predictions. For this metabolic adaptation, PknB was found to interact with CdaA and the yvcK/glmR regulon. The involved GlmR structure and the GlmS riboswitch were modelled. Furthermore, PknB phosphorylation lowered the expression of many virulence factors, and the study shed light on S. aureus infection processes.

Fatty acids can inhibit Staphylococcus aureus SaeS activity at the membrane independent of alterations in respiration

In Staphylococcus aureus, the two-component system SaeRS is responsible for regulating various virulence factors essential for the success of this pathogen. SaeRS can be stimulated by neutrophil-derived products but has also recently been shown to be inactivated by the presence of free fatty acids. A mechanism for how fatty acids negatively impacts SaeRS has not been described. We found that unsaturated fatty acids, as well as fatty acids not commonly found in Staphylococcal membranes, prevent the activation of SaeRS at a lower concentration than their saturated counterparts. These fatty acids can negatively impact SaeRS without altering the respiratory capacity of the bacterium. To uncover a potential mechanism for how fatty acids impact SaeRS function/activity, we utilized a naturally occurring point mutation found in S. aureus as well as chimeric SaeS proteins. Using these tools, we identified that the native transmembrane domains of SaeS dictate the transcriptional response to fatty acids in S. aureus. Our data support a model where free fatty acids alter the activity of the two-component system SaeRS directly through the sensor kinase SaeS and is dependent on the transmembrane domains of the protein.

Regulation of virulence and β-lactamase gene expression in Staphylococcus aureus isolates: cooperation of two-component systems in bloodstream superbugs

BACKGROUND

Methicillin-resistant Staphylococcus aureus (MRSA)-bloodstream infections (BSI) are predominantly seen in the hospital or healthcare-associated host. Nevertheless, the interactions of virulence factor (VFs) regulators and β-lactam resistance in MRSA-BSI are unclear. This study aims to characterize the molecular relationship of two-component systems of VFs and the expression of the β-lactamase gene in MRSA-BSI isolates. In this study, 639 samples were collected from BSI and identified by phenotypic methods. We performed extensive molecular characterization, including SCCmec type, agr type, VFs gene profiles determinations, and MLST on isolates. Also, a quantitative real-time PCR (q-RT PCR) assay was developed for identifying the gene expressions.

RESULTS

Ninety-one (91) S. aureus and 61 MRSA (67.0%) strains were detected in BSI samples. The presence of VFs and SCCmec genes in MRSA isolates were as follows: tst (31.4%), etA (18.0%), etB (8.19%), lukS-PVL (31.4%), lukF-PV (18.0%), lukE-lukD (16.3%), edin (3.2%), hla (16.3%), hlb (18.0%), hld (14.7%), hlg (22.9%), SCCmecI (16.3%), SCCmecII (22.9%), SCCmecIII (36.0%), SCCmecIV (21.3%), and SCCmecV (16.3%). Quantitative real-time PCR showed overexpression of mecRI and mecI in the toxigenic isolates. Moreover, RNAIII and sarA genes were the highest expressions of MRSA strains. The multi-locus sequence typing data confirmed a high prevalence of CC5, CC8, and CC30. However, ST30, ST22, and ST5 were the most prevalent in the resistant and toxigenic strains.

CONCLUSION

We demonstrated that although regulation of β-lactamase gene expressions is a significant contributor to resistance development, two-component systems also influence antibiotic resistance development in MRSA-BSI isolates. This indicates that resistant strains might have pathogenic potential. We also confirmed that some MLST types are more successful colonizers with a potential for MRSA-BSI.

Influence of sub-inhibitory concentrations of antimicrobials on micrococcal nuclease and biofilm formation in Staphylococcus aureus

A major contributor to biomaterial associated infection (BAI) is Staphylococcus aureus. This pathogen produces a protective biofilm, making eradication difficult. Biofilms are composed of bacteria encapsulated in a matrix of extracellular polymeric substances (EPS) comprising polysaccharides, proteins and extracellular DNA (eDNA). S. aureus also produces micrococcal nuclease (MN), an endonuclease which contributes to biofilm composition and dispersion, mainly expressed by nuc1. MN expression can be modulated by sub-minimum inhibitory concentrations of antimicrobials. We investigated the relation between the biofilm and MN expression and the impact of the application of antimicrobial pressure on this relation. Planktonic and biofilm cultures of three S. aureus strains, including a nuc1 deficient strain, were cultured under antimicrobial pressure. Results do not confirm earlier findings that MN directly influences total biomass of the biofilm but indicated that nuc1 deletion stimulates the polysaccharide production per CFU in the biofilm in in vitro biofilms. Though antimicrobial pressure of certain antibiotics resulted in significantly increased quantities of polysaccharides per CFU, this did not coincide with significantly reduced MN activity. Erythromycin and resveratrol significantly reduced MN production per CFU but did not affect total biomass or biomass/CFU. Reduction of MN production may assist in the eradication of biofilms by the host immune system in clinical situations.

In-Depth Characterization of the Staphylococcus aureus Phosphoproteome Reveals New Targets of Stk1

Staphylococcus aureus is a major cause of infections worldwide, and infection results in a variety of diseases. As of no surprise, protein phosphorylation is an important game player in signaling cascades and has been shown to be involved in S. aureus virulence. Albeit long neglected, eukaryotic-type serine/threonine kinases in S. aureus have been implicated in this complex signaling cascades. Due to the substoichiometric nature of protein phosphorylation and a lack of suitable analysis tools, the knowledge of these cascades is, however, to date, still limited. Here, were apply an optimized protocol for efficient phosphopeptide enrichment via Fe3+-IMAC followed by LC-MS/MS to get a better understanding of the impact of protein phosphorylation on the complex signaling networks involved in pathogenicity. By profiling a serine/threonine kinase and phosphatase mutant from a methicillin-resistant S. aureus mutant library, we generated the most comprehensive phosphoproteome data set of S. aureus to date, aiding a better understanding of signaling in bacteria. With the identification of 3800 class I p-sites, we were able to increase the number of identifications by more than 21 times compared with recent literature. In addition, we were able to identify 74 downstream targets of the only reported eukaryotic-type Ser/Thr kinase of the S. aureus strain USA300, Stk1. This work allowed an extensive analysis of the bacterial phosphoproteome and indicates that Ser/Thr kinase signaling is far more abundant than previously anticipated in S. aureus.

Staphylococcus aureus Biofilms and Their Response to a Relevant in vivo Iron Source

Biofilm infections can be chronic, life threatening and challenging to eradicate. Understanding in vivo stimuli affecting the biofilm cycle is one step toward targeted prevention strategies. Iron restriction by the host is a stimulus for biofilm formation for some Staphylococcus aureus isolates; however, in some infection scenarios bacteria are exposed to abundant amounts of hemoglobin (Hb), which S. aureus is able to use as iron source. Thus, we hypothesized a role for Hb in the biofilm infection. Microplate “biofilm” assays showed biofilm-matrix production was increased in the presence of hemoglobin when compared to the provision of iron as an inorganic salt. Microscopic analysis of biofilms showed that the provision of iron as hemoglobin consistently caused thicker and more structured biofilms when compared to the effect of the inorganic iron source. Iron responsive biofilm gene expression analysis showed that Agr Quorum Sensing, a known biofilm dispersal marker, was repressed with hemoglobin but induced with an equivalent amount of inorganic iron in the laboratory strain Newman. The gene expression of two biofilm structuring agents, PSMα and PSMβ, differed in the response to the iron source provided and was not correlated to hemoglobin-structured biofilms. A comparison of the model pathogen S. aureus Newman with local clinical isolates demonstrated that while there was a similar phenotypic biofilm response to hemoglobin, there was substantial variation in the expression of key biofilm dispersal markers, suggesting an underappreciated variation in biofilm regulome among S. aureus isolates and that no general inferences can be made by studying the behavior of single strains.

Staphylococcal Biofilm Development: Structure, Regulation, and Treatment Strategies

In many natural and clinical settings, bacteria are associated with some type of biotic or abiotic surface that enables them to form biofilms, a multicellular lifestyle with bacteria embedded in an extracellular matrix. Staphylococcus aureus and Staphylococcus epidermidis, the most frequent causes of biofilm-associated infections on indwelling medical devices, can switch between an existence as single free-floating cells and multicellular biofilms. During biofilm formation, cells first attach to a surface and then multiply to form microcolonies. They subsequently produce the extracellular matrix, a hallmark of biofilm formation, which consists of polysaccharides, proteins, and extracellular DNA. After biofilm maturation into three-dimensional structures, the biofilm community undergoes a disassembly process that leads to the dissemination of staphylococcal cells. As biofilms are dynamic and complex biological systems, staphylococci have evolved a vast network of regulatory mechanisms to modify and fine-tune biofilm development upon changes in environmental conditions. Thus, biofilm formation is used as a strategy for survival and persistence in the human host and can serve as a reservoir for spreading to new infection sites. Moreover, staphylococcal biofilms provide enhanced resilience toward antibiotics and the immune response and impose remarkable therapeutic challenges in clinics worldwide. This review provides an overview and an updated perspective on staphylococcal biofilms, describing the characteristic features of biofilm formation, the structural and functional properties of the biofilm matrix, and the most important mechanisms involved in the regulation of staphylococcal biofilm formation. Finally, we highlight promising strategies and technologies, including multitargeted or combinational therapies, to eradicate staphylococcal biofilms.

Staphylococcal Biofilm Development: Structure, Regulation, and Treatment Strategies

In many natural and clinical settings, bacteria are associated with some type of biotic or abiotic surface that enables them to form biofilms, a multicellular lifestyle with bacteria embedded in an extracellular matrix. Staphylococcus aureus and Staphylococcus epidermidis, the most frequent causes of biofilm-associated infections on indwelling medical devices, can switch between an existence as single free-floating cells and multicellular biofilms. During biofilm formation, cells first attach to a surface and then multiply to form microcolonies. They subsequently produce the extracellular matrix, a hallmark of biofilm formation, which consists of polysaccharides, proteins, and extracellular DNA. After biofilm maturation into three-dimensional structures, the biofilm community undergoes a disassembly process that leads to the dissemination of staphylococcal cells. As biofilms are dynamic and complex biological systems, staphylococci have evolved a vast network of regulatory mechanisms to modify and fine-tune biofilm development upon changes in environmental conditions. Thus, biofilm formation is used as a strategy for survival and persistence in the human host and can serve as a reservoir for spreading to new infection sites. Moreover, staphylococcal biofilms provide enhanced resilience toward antibiotics and the immune response and impose remarkable therapeutic challenges in clinics worldwide. This review provides an overview and an updated perspective on staphylococcal biofilms, describing the characteristic features of biofilm formation, the structural and functional properties of the biofilm matrix, and the most important mechanisms involved in the regulation of staphylococcal biofilm formation. Finally, we highlight promising strategies and technologies, including multitargeted or combinational therapies, to eradicate staphylococcal biofilms.

N-Nonyloxypentyl-l-Deoxynojirimycin Inhibits Growth, Biofilm Formation and Virulence Factors Expression of Staphylococcus aureus

Staphylococcus aureus is one of the major causes of hospital- and community-associated bacterial infections throughout the world, which are difficult to treat due to the rising number of drug-resistant strains. New molecules displaying potent activity against this bacterium are urgently needed. In this study, d- and l-deoxynojirimycin (DNJ) and a small library of their N-alkyl derivatives were screened against S. aureus ATCC 29213, with the aim to identify novel candidates with inhibitory potential. Among them, N-nonyloxypentyl-l-DNJ (l-NPDNJ) proved to be the most active compound against S. aureus ATCC 29213 and its clinical isolates, with the minimum inhibitory concentration (MIC) value of 128 μg/mL. l-NPDNJ also displayed an additive effect with gentamicin and oxacillin against the gentamicin- and methicillin-resistant S. aureus isolate 00717. Sub-MIC values of l-NPDNJ affected S. aureus biofilm development in a dose-dependent manner, inducing a strong reduction in biofilm biomass. Moreover, real-time reverse transcriptase PCR analysis revealed that l-NPDNJ effectively inhibited at sub-MIC values the transcription of the spa, hla, hlb and sea virulence genes, as well as the agrA and saeR response regulator genes.

Isolating Crucial Steps in Induction of Infective Endocarditis With Preclinical Modeling of Host Pathogen Interaction

Animal models of Staphylococcus aureus infective endocarditis (IE), especially in rodents, are commonly used to investigate the underlying pathogenesis, disease progression, potential diagnostic approaches, and therapeutic treatment. All these models are based on surgical interventions, and imply valve trauma by placing a polyurethane catheter at the aortic root. While the influence of endothelial damage and inflammation on the induction of IE has been studied intensively, the role of the catheter, as permanent source of bacteremia, and the interplay with bacterial virulence factors during the formation of IE is poorly understood. In our study, we aimed at identifying which set of preconditions is required for induction and formation of IE: (1) tissue injury, (2) permanent presence of bacteria, and (3) presence of the full bacterial repertoire of adhesion proteins. We investigated the manifestation of the disease in different modifications of the animal model, considering different degrees of endothelial damage and the presence or absence of the catheter. In four infection models the induction of IE was assessed by using two bacterial strains with different expression patterns of virulence factors – S. aureus 6850 and Newman. In vivo magnetic resonance imaging showed conspicuous morphological structures on the aortic valves, when an endothelial damage and a continuous bacterial source were present simultaneously. Cellular and inflammatory pathophysiology were characterized additionally by histology, real-time quantitative polymerase chain reaction analysis, and bacterial counts, revealing strain-specific pathogenesis and manifestation of IE, crucially influenced by bacterial adherence and toxicity. The severity of IE was dependent on the degree of endothelial irritation. However, even severe endothelial damage in the absence of a permanent bacterial source resulted in reduced valve infection. The spread of bacteria to other organs was also dependent on the pathogenic profile of the infectious agent.

PYED-1 Inhibits Biofilm Formation and Disrupts the Preformed Biofilm of Staphylococcus aureus

Pregnadiene-11-hydroxy-16α,17α-epoxy-3,20-dione-1 (PYED-1), a heterocyclic corticosteroid derivative of deflazacort, exhibits broad-spectrum antibacterial activity against Gram-negative and Gram-positive bacteria. Here, we investigated the effect of PYED-1 on the biofilms of Staphylococcus aureus, an etiological agent of biofilm-based chronic infections such as osteomyelitis, indwelling medical device infections, periodontitis, chronic wound infections, and endocarditis. PYED-1 caused a strong reduction in biofilm formation in a concentration dependent manner. Furthermore, it was also able to completely remove the preformed biofilm. Transcriptional analysis performed on the established biofilm revealed that PYED-1 downregulates the expression of genes related to quorum sensing (agrA, RNAIII, hld, psm, and sarA), surface proteins (clfB and fnbB), secreted toxins (hla, hlb, and lukD), and capsular polysaccharides (capC). The expression of genes that encode two main global regulators, sigB and saeR, was also significantly inhibited after treatment with PYED-1. In conclusion, PYED-1 not only effectively inhibited biofilm formation, but also eradicated preformed biofilms of S. aureus, modulating the expression of genes related to quorum sensing, surface and secreted proteins, and capsular polysaccharides. These results indicated that PYED-1 may have great potential as an effective antibiofilm agent to prevent S. aureus biofilm-associated infections.

The Accessory Gene saeP of the SaeR/S Two-Component Gene Regulatory System Impacts Staphylococcus aureus Virulence During Neutrophil Interaction

Staphylococcus aureus (S. aureus) causes a range of diseases ranging from superficial skin and soft-tissue infections to invasive and life-threatening conditions (Klevens et al., 2007; Kobayashi et al., 2015). S. aureus utilizes the Sae sensory system to adapt to neutrophil challenge. Although the roles of the SaeR response regulator and its cognate sensor kinase SaeS have been demonstrated to be critical for surviving neutrophil interaction and for causing infection, the roles for the accessory proteins SaeP and SaeQ remain incompletely defined. To characterize the functional role of these proteins during innate immune interaction, we generated isogenic deletion mutants lacking these accessory genes in USA300 (USA300ΔsaeP and USA300ΔsaeQ). S. aureus survival was increased following phagocytosis of USA300ΔsaeP compared to USA300 by neutrophils. Additionally, secreted extracellular proteins produced by USA300ΔsaeP cells caused significantly more plasma membrane damage to human neutrophils than extracellular proteins produced by USA300 cells. Deletion of saeQ resulted in a similar phenotype, but effects did not reach significance during neutrophil interaction. The enhanced cytotoxicity of USA300ΔsaeP cells toward human neutrophils correlated with an increased expression of bi-component leukocidins known to target these immune cells. A saeP and saeQ double mutant (USA300ΔsaePQ) showed a significant increase in survival following neutrophil phagocytosis that was comparable to the USA300ΔsaeP single mutant and increased the virulence of USA300 during murine bacteremia. These data provide evidence that SaeP modulates the Sae-mediated response of S. aureus against human neutrophils and suggest that saeP and saeQ together impact pathogenesis in vivo.

Staphylococcus aureus Fibronectin Binding Protein A Mediates Biofilm Development and Infection

Implanted medical device-associated infections pose significant health risks, as they are often the result of bacterial biofilm formation. Staphylococcus aureus is a leading cause of biofilm-associated infections which persist due to mechanisms of device surface adhesion, biofilm accumulation, and reprogramming of host innate immune responses. We found that the S. aureus fibronectin binding protein A (FnBPA) is required for normal biofilm development in mammalian serum and that the SaeRS two-component system is required for functional FnBPA activity in serum. Furthermore, serum-developed biofilms deficient in FnBPA were more susceptible to macrophage invasion, and in a model of biofilm-associated implant infection, we found that FnBPA is crucial for the establishment of infection. Together, these findings show that S. aureus FnBPA plays an important role in physical biofilm development and represents a potential therapeutic target for the prevention and treatment of device-associated infections.

Role of respiratory NADH oxidation in the regulation of Staphylococcus aureus virulence

The success of Staphylococcus aureus as a pathogen is due to its capability of fine-tuning its cellular physiology to meet the challenges presented by diverse environments, which allows it to colonize multiple niches within a single vertebrate host. Elucidating the roles of energy-yielding metabolic pathways could uncover attractive therapeutic strategies and targets. In this work, we seek to determine the effects of disabling NADH-dependent aerobic respiration on the physiology of S. aureus. Differing from many pathogens, S. aureus has two type-2 respiratory NADH dehydrogenases (NDH-2s) but lacks the respiratory ion-pumping NDHs. Here, we show that the NDH-2s, individually or together, are not essential either for respiration or growth. Nevertheless, their absence eliminates biofilm formation, production of α-toxin, and reduces the ability to colonize specific organs in a mouse model of systemic infection. Moreover, we demonstrate that the reason behind these phenotypes is the alteration of the fatty acid metabolism. Importantly, the SaeRS two-component system, which responds to fatty acids regulation, is responsible for the link between NADH-dependent respiration and virulence in S. aureus.

Stochastic Expression of Sae-Dependent Virulence Genes during Staphylococcus aureus Biofilm Development Is Dependent on SaeS

The intricate process of biofilm formation in the human pathogen Staphylococcus aureus involves distinct stages during which a complex mixture of matrix molecules is produced and modified throughout the developmental cycle. Early in biofilm development, a subpopulation of cells detaches from its substrate in an event termed "exodus" that is mediated by SaePQRS-dependent stochastic expression of a secreted staphylococcal nuclease, which degrades extracellular DNA within the matrix, causing the release of cells and subsequently allowing for the formation of metabolically heterogenous microcolonies. Since the SaePQRS regulatory system is involved in the transcriptional control of multiple S. aureus virulence factors, the expression of several additional virulence genes was examined within a developing biofilm by introducing fluorescent gene reporter plasmids into wild-type S. aureus and isogenic regulatory mutants and growing these strains in a microfluidic system that supplies the bacteria with a constant flow of media while simultaneously imaging developing biofilms in 5-min intervals. This study demonstrated that multiple virulence genes, including nuc, were expressed stochastically within a specialized subpopulation of cells in nascent biofilms. We demonstrated that virulence genes regulated by SaePQRS were stochastically expressed in nearly all strains examined whereas Agr-regulated genes were expressed more homogenously within maturing microcolonies. The commonly used Newman strain contains a variant of SaeS (SaeS^P) that confers constitutive kinase activity to the protein and caused this strain to lack the stochastic expression pattern observed in other strain backgrounds. Importantly, repair of the SaeS^P allele resulting in reversion to the well-conserved SaeS ^L allele found in other strains restored stochastic expression in this strain.IMPORTANCE Staphylococcus aureus is an important human pathogen capable of colonizing diverse tissue types and inducing severe disease in both immunocompromised and otherwise healthy individuals. Biofilm infections caused by this bacterial species are of particular concern because of their persistence, even in the face of intensive therapeutic intervention. The results of the current study demonstrate the stochastic nature of Sae-mediated virulence gene expression in S. aureus and indicate that this regulatory system may function as a "bistable switch" in a manner similar to that seen with regulators controlling competence gene expression in Bacillus subtilis and persister cell formation in Escherichia coli The results of this study provide a new perspective on the complex mechanisms utilized by S. aureus during the establishment of infections.

Contribution of hla Regulation by SaeR to Staphylococcus aureus USA300 Pathogenesis

The SaeRS two-component system in Staphylococcus aureus is critical for regulation of many virulence genes, including hla, which encodes alpha-toxin. However, the impact of regulation of alpha-toxin by Sae on S. aureus pathogenesis has not been directly addressed. Here, we mutated the SaeR-binding sequences in the hla regulatory region and determined the contribution of this mutation to hla expression and pathogenesis in strain USA300 JE2. Western blot analyses revealed drastic reduction of alpha-toxin levels in the culture supernatants of SaeR-binding mutant in contrast to the marked alpha-toxin production in the wild type. The SaeR-binding mutation had no significant effect on alpha-toxin regulation by Agr, MgrA, and CcpA. In animal studies, we found that the SaeR-binding mutation did not contribute to USA300 JE2 pathogenesis using a rat infective endocarditis model. However, in a rat skin and soft tissue infection model, the abscesses on rats infected with the mutant were significantly smaller than the abscesses on those infected with the wild type but similar to the abscesses on those infected with a saeR mutant. These studies indicated that there is a direct effect of hla regulation by SaeR on pathogenesis but that the effect depends on the animal model used.

Revisiting the regulation of the capsular polysaccharide biosynthesis gene cluster in Staphylococcus aureus

Capsular polysaccharide (CP) biosynthesis in Staphylococcus aureus is tightly controlled resulting in a heterogeneous phenotype within a population and CP being mainly detectable in nongrowing cells. Expression of the corresponding biosynthesis gene cluster is driven by one promoter element (P_cap ). Here, we demonstrate that P_cap contains a main SigB-dependent promoter. The SigB consensus motif overlaps with a previously described inverted repeat (IR) that is crucial for cap expression. The essentiality of the IR is derived from this region acting as a SigB binding site rather than as an operator site for the proposed cap activators RbsR and MsaB. Furthermore, P_cap contains an extensive upstream region harboring a weak SigA-dependent promoter and binding sites for cap repressors such as SaeR, CodY and Rot. Heterogeneous CP synthesis is determined by SigB activity and repressor binding to the upstream region. SigB dependency and regulation by the upstream repressors are also sufficient to explain the temporal gene expression pattern at the transcriptional level. However, CP synthesis remains growth phase-dependent even when transcription is rendered constitutive, suggesting additional posttranscriptional regulatory circuits. Thus, the interference of multiple repressors with SigB-dependent promoter activity as well as post-transcriptional mechanisms ensure the appropriate regulation of CP synthesis.

The ClpCP Complex Modulates Respiratory Metabolism in Staphylococcus aureus and Is Regulated in a SrrAB-Dependent Manner

The staphylococcal respiratory regulator (SrrAB) modulates energy metabolism in Staphylococcus aureus Studies have suggested that regulated protein catabolism facilitates energy homeostasis. Regulated proteolysis in S. aureus is achieved through protein complexes composed of a peptidase (ClpQ or ClpP) in association with an AAA⁺ family ATPase (typically, ClpC or ClpX). In the present report, we tested the hypothesis that SrrAB regulates a Clp complex to facilitate energy homeostasis in S. aureus Strains deficient in one or more Clp complexes were attenuated for growth in the presence of puromycin, which causes enrichment of misfolded proteins. A ΔsrrAB strain had increased sensitivity to puromycin. Epistasis experiments suggested that the puromycin sensitivity phenotype of the ΔsrrAB strain was a result of decreased ClpC activity. Consistent with this, transcriptional activity of clpC was decreased in the ΔsrrAB mutant, and overexpression of clpC suppressed the puromycin sensitivity of the ΔsrrAB strain. We also found that ClpC positively influenced respiration and that it did so upon association with ClpP. In contrast, ClpC limited fermentative growth, while ClpP was required for optimal fermentative growth. Metabolomics studies demonstrated that intracellular metabolic profiles of the ΔclpC and ΔsrrAB mutants were distinct from those of the wild-type strain, supporting the notion that both ClpC and SrrAB affect central metabolism. We propose a model wherein SrrAB regulates energy homeostasis, in part, via modulation of regulated proteolysis.IMPORTANCE Oxygen is used as a substrate to derive energy by the bacterial pathogen Staphylococcus aureus during infection; however, S. aureus can also grow fermentatively in the absence of oxygen. To successfully cause infection, S. aureus must tailor its metabolism to take advantage of respiratory activity. Different proteins are required for growth in the presence or absence of oxygen; therefore, when cells transition between these conditions, several proteins would be expected to become unnecessary. In this report, we show that regulated proteolysis is used to modulate energy metabolism in S. aureus We report that the ClpCP protein complex is involved in specifically modulating aerobic respiratory growth but is dispensable for fermentative growth.

An intramembrane sensory circuit monitors sortase A-mediated processing of streptococcal adhesins

Bacterial adhesins mediate adhesion to substrates and biofilm formation. Adhesins of the LPXTG family are posttranslationally processed by the cell membrane-localized peptidase sortase A, which cleaves the LPXTG motif. This generates a short C-terminal peptide (C-pep) that remains in the cell membrane, whereas the mature adhesin is incorporated into the cell wall. Genes encoding adhesins of the oral bacterium Streptococcus gordonii were differentially expressed depending on whether the bacteria were isolated from saliva or dental plaque and appeared to be coordinately regulated. Deletion of sspA and sspB (sspAB), both of which encode LPXTG-containing adhesins, unexpectedly enhanced adhesion and biofilm formation. C-peps produced from a model LPXTG-containing adhesin localized to the cell membrane and bound to and inhibited the intramembrane sensor histidine kinase SGO_1180, thus preventing activation of the cognate response regulator SGO_1181. The absence of SspAB C-peps induced the expression of the scaCBA operon encoding the lipoprotein adhesin ScaA, which was sufficient to preserve and even enhance biofilm formation. This C-pep-driven regulatory circuit also exists in pathogenic streptococci and is likely conserved among Gram-positive bacteria. This quality control mechanism ensures that the bacteria can form biofilms under diverse environmental conditions and may play a role in optimizing adhesion and biofilm formation.

Regulation of Staphylococcus aureus Virulence

Staphylococcus aureus is a Gram-positive opportunistic pathogen that has evolved a complex regulatory network to control virulence. One of the main functions of this interconnected network is to sense various environmental cues and respond by altering the production of virulence factors necessary for survival in the host, including cell surface adhesins and extracellular enzymes and toxins. Of these S. aureus regulatory systems, one of the best studied is the accessory gene regulator (agr), which is a quorum-sensing system that senses the local concentration of a cyclic peptide signaling molecule. This system allows S. aureus to sense its own population density and translate this information into a specific gene expression pattern. Besides agr, this pathogen uses other two-component systems to sense specific cues and coordinates responses with cytoplasmic regulators of the SarA protein family and alternative sigma factors. These divergent regulatory systems integrate the various environmental and host-derived signals into a network that ensures optimal pathogen response to the changing conditions. This article gives an overview of the most important and best-studied S. aureus regulatory systems and summarizes the functions of these regulators during host interactions. The regulatory systems discussed include the agr quorum-sensing system; the SaeRS, SrrAB, and ArlRS two-component systems, the cytoplasmic SarA-family regulators (SarA, Rot, and MgrA); and the alternative sigma factors (SigB and SigH).

Redundant and Distinct Roles of Secreted Protein Eap and Cell Wall-Anchored Protein SasG in Biofilm Formation and Pathogenicity of Staphylococcus aureus

Chronic and fatal infections caused by Staphylococcus aureus are sometimes associated with biofilm formation. Secreted proteins and cell wall-anchored proteins (CWAPs) are important for the development of polysaccharide-independent biofilms, but functional relationships between these proteins are unclear. In the present study, we report the roles of the extracellular adherence protein Eap and the surface CWAP SasG in S. aureus MR23, a clinical methicillin-resistant isolate that forms a robust protein-dependent biofilm and accumulates a large amount of Eap in the extracellular matrix. Double deletion of eap and sasG, but not single eap or sasG deletion, reduced the biomass of the formed biofilm. Mutational analysis demonstrated that cell wall anchorage is essential for the role of SasG in biofilm formation. Confocal laser scanning microscopy revealed that MR23 formed a rugged and thick biofilm; deletion of both eap and sasG reduced biofilm ruggedness and thickness. Although sasG deletion did not affect either of these features, eap deletion reduced the ruggedness but not the thickness of the biofilm. This indicated that Eap contributes to the rough irregular surface structure of the MR23 biofilm and that both Eap and SasG play roles in biofilm thickness. The level of pathogenicity of the Δeap ΔsasG strain in a silkworm larval infection model was significantly lower (P < 0.05) than those of the wild type and single-deletion mutants. Collectively, these findings highlight the redundant and distinct roles of a secreted protein and a CWAP in biofilm formation and pathogenicity of S. aureus and may inform new strategies to control staphylococcal biofilm infections.

Redundant and Distinct Roles of Secreted Protein Eap and Cell Wall-Anchored Protein SasG in Biofilm Formation and Pathogenicity of Staphylococcus aureus

Chronic and fatal infections caused by Staphylococcus aureus are sometimes associated with biofilm formation. Secreted proteins and cell wall-anchored proteins (CWAPs) are important for the development of polysaccharide-independent biofilms, but functional relationships between these proteins are unclear. In the present study, we report the roles of the extracellular adherence protein Eap and the surface CWAP SasG in S. aureus MR23, a clinical methicillin-resistant isolate that forms a robust protein-dependent biofilm and accumulates a large amount of Eap in the extracellular matrix. Double deletion of eap and sasG, but not single eap or sasG deletion, reduced the biomass of the formed biofilm. Mutational analysis demonstrated that cell wall anchorage is essential for the role of SasG in biofilm formation. Confocal laser scanning microscopy revealed that MR23 formed a rugged and thick biofilm; deletion of both eap and sasG reduced biofilm ruggedness and thickness. Although sasG deletion did not affect either of these features, eap deletion reduced the ruggedness but not the thickness of the biofilm. This indicated that Eap contributes to the rough irregular surface structure of the MR23 biofilm and that both Eap and SasG play roles in biofilm thickness. The level of pathogenicity of the Δeap ΔsasG strain in a silkworm larval infection model was significantly lower (P < 0.05) than those of the wild type and single-deletion mutants. Collectively, these findings highlight the redundant and distinct roles of a secreted protein and a CWAP in biofilm formation and pathogenicity of S. aureus and may inform new strategies to control staphylococcal biofilm infections.

A Fluorescence-based Method to Study Bacterial Gene Regulation in Infected Tissues

Bacterial virulence genes are often regulated at the transcriptional level by multiple factors that respond to different environmental signals. Some factors act directly on virulence genes; others control pathogenesis by adjusting the expression of downstream regulators or the accumulation of signals that affect regulator activity. While regulation has been studied extensively during in vitro growth, relatively little is known about how gene expression is adjusted during infection. Such information is important when a particular gene product is a candidate for therapeutic intervention. Transcriptional approaches like quantitative, real-time RT-PCR and RNA-Seq are powerful ways to examine gene expression on a global level but suffer from many technical challenges including low abundance of bacterial RNA compared to host RNA, and sample degradation by RNases. Evaluating regulation using fluorescent reporters is relatively easy and can be multiplexed with fluorescent proteins with unique spectral properties. The method allows for single-cell, spatiotemporal analysis of gene expression in tissues that exhibit complex three-dimensional architecture and physiochemical gradients that affect bacterial regulatory networks. Such information is lost when data are averaged over the bulk population. Herein, we describe a method for quantifying gene expression in bacterial pathogens in situ. The method is based on simple tissue processing and direct observation of fluorescence from reporter proteins. We demonstrate the utility of this system by examining the expression of Staphylococcus aureus thermonuclease (nuc), whose gene product is required for immune evasion and full virulence ex vivo and in vivo. We show that nuc-gfp is strongly expressed in renal abscesses and reveal heterogeneous gene expression due in part to apparent spatial regulation of nuc promoter activity in abscesses fully engaged with the immune response. The method can be applied to any bacterium with a manipulatable genetic system and any infection model, providing valuable information for preclinical studies and drug development.

Aspartic Acid Residue 51 of SaeR Is Essential for Staphylococcus aureus Virulence

Staphylococcus aureus is a common Gram-positive bacteria that is a major cause of human morbidity and mortality. The SaeR/S two-component sensory system of S. aureus is important for virulence gene transcription and pathogenesis. However, the influence of SaeR phosphorylation on virulence gene transcription is not clear. To determine the importance of potential SaeR phosphorylation sites for S. aureus virulence, we generated genomic alanine substitutions at conserved aspartic acid residues in the receiver domain of the SaeR response regulator in clinically significant S. aureus pulsed-field gel electrophoresis (PFGE) type USA300. Transcriptional analysis demonstrated a dramatic reduction in the transcript abundance of various toxins, adhesins, and immunomodulatory proteins for SaeR with an aspartic acid to alanine substitution at residue 51. These findings corresponded to a significant decrease in cytotoxicity against human erythrocytes and polymorphonuclear leukocytes, the ability to block human myeloperoxidase activity, and pathogenesis during murine soft-tissue infection. Analysis of SaeR sequences from over 8,000 draft S. aureus genomes revealed that aspartic acid residue 51 is 100% conserved. Collectively, these results demonstrate that aspartic acid residue 51 of SaeR is essential for S. aureus virulence and underscore a conserved target for novel antimicrobial strategies that treat infection caused by this pathogen.

Staphylococcus aureus coagulases are exploitable yet stable public goods in clinically relevant conditions

Coagulation is an innate defense mechanism intended to limit blood loss and trap invading pathogens during infection. However, Staphylococcus aureus has the ability to hijack the coagulation cascade and generate clots via secretion of coagulases. Although many S. aureus have this characteristic, some do not. The population dynamics regarding this defining trait have yet to be explored. We report here that coagulases are public goods that confer protection against antimicrobials and immune factors within a local population or community, thus promoting growth and virulence. By utilizing variants of a methicillin-resistant S. aureus we infer that the secretion of coagulases is a cooperative trait, which is subject to exploitation by invading mutants that do not produce the public goods themselves. However, overexploitation, "tragedy of the commons," does not occur at clinically relevant conditions. Our micrographs indicate this is due to spatial segregation and population viscosity. These findings emphasize the critical role of coagulases in a social evolution context and provide a possible explanation as to why the secretion of these public goods is maintained in mixed S. aureus communities.

Long Noncoding RNA SSR42 Controls Staphylococcus aureus Alpha-Toxin Transcription in Response to Environmental Stimuli

Staphylococcus aureus is a human pathogen causing a variety of diseases by versatile expression of a large set of virulence factors that most prominently features the cytotoxic and hemolytic pore-forming alpha-toxin. Expression of alpha-toxin is regulated by an intricate network of transcription factors. These include two-component systems sensing quorum and environmental signals as well as regulators reacting to the nutritional status of the pathogen. We previously identified the repressor of surface proteins (Rsp) as a virulence regulator. Acute cytotoxicity and hemolysis are strongly decreased in rsp mutants, which are characterized by decreased transcription of toxin genes as well as loss of transcription of a 1,232-nucleotide (nt)-long noncoding RNA (ncRNA), SSR42. Here, we show that SSR42 is the effector of Rsp in transcription regulation of the alpha-toxin gene, hla SSR42 transcription is enhanced after exposure of S. aureus to subinhibitory concentrations of oxacillin which thus leads to an SSR42-dependent increase in hemolysis. Aside from Rsp, SSR42 transcription is under the control of additional global regulators, such as CodY, AgrA, CcpE, and σB, but is positioned upstream of the two-component system SaeRS in the regulatory cascade leading to alpha-toxin production. Thus, alpha-toxin expression depends on two long ncRNAs, SSR42 and RNAIII, which control production of the cytolytic toxin on the transcriptional and translational levels, respectively, with SSR42 as an important regulator of SaeRS-dependent S. aureus toxin production in response to environmental and metabolic signals.IMPORTANCEStaphylococcus aureus is a major cause of life-threatening infections. The bacterium expresses alpha-toxin, a hemolysin and cytotoxin responsible for many of the pathologies of S. aureus Alpha-toxin production is enhanced by subinhibitory concentrations of antibiotics. Here, we show that this process is dependent on the long noncoding RNA, SSR42. Further, SSR42 itself is regulated by several global regulators, thereby integrating environmental and nutritional signals that modulate hemolysis of the pathogen.